<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div id="app">
        <button v-on:click="add">+</button>
        <button>{{num}}</button>
        <button v-on:click="sub">-</button>
    </div>
</body>
<script src="https://cn.vuejs.org/js/vue.js"></script>
<script>
    let vm = new Vue({
        el:"#app",
        data:{
            num:0
        },
        methods: {
            add:function(e){
                this.num++
            },
            sub:function(e){
                if(this.num-1<0){
                    alert("再减你给我啊？？？");
                }else{
                    this.num--;
                }
            }
        },
    })
</script>
</html>